1. Field of the Invention
The present invention relates to data communication system and apparatus for performing data communication by using a public telephone line network.
2. Related Background Art
Hitherto, there has been known a public telephone line network for transmitting accounting pulses from the network side to a terminal during the use of the line in order to properly inform a user of line charges or to collect the line charges from a user. The public telephone line is generally used for voice communication.
In recent years, even in data terminal equipment such as facsimile apparatus to communicate image data, or handy-type terminal to communicate code data, etc., data communication is executed with partner terminal equipment by connecting to the public telephone line.
To perform the data communication by using the public telephone network, it is necessary to call a communication partner to form a communication path and, thereafter, to perform the data communication with the communication partner called. The communication path (speech path) is formed by switching equipment of the line network. The line charges are added (metering system) in accordance with the distance of the communication path and use time.
In the case of using an electromagnetic exchange or the like, an electromagnetic counter which operates by DC pulses is used as a call meter for counting the charges. An accounting pulse is output every metering and the call meter is counted. Therefore, as shown in FIG. 5, the accounting pulse is produced by inverting the DC voltage polarity of the line.
However, since the accounting pulses are transmitted from the network to a terminal through the communication line during the communication, a noise electric power is generated during the operations of inverting the polarity. In the case of the speech communication, such a noise electric power is not relatively conscious. However, in the case of the data communication, the noise electric power also becomes a cause of the generation of transmission errors. That is, the noise electric power is generated in the intervals between a and b and between c and d in FIG. 5, so that the data transmission quality deteriorates. Particularly, when data communication over a long distance is performed, the accounting pulses are frequently generated, causing a large obstacle to improve the error rate.
On the other hand, there is a drawback such that since data is retransmitted for every generation of an error, the communication time becomes long.